Core construction for current-limiting fuse

ABSTRACT

A current-limiting fuse structure comprising a generally tubular, electrically insulating casing having terminal means disposed adjacent to the opposite ends thereof. An electrically insulating core is disposed in the casing with opposite ends adjacent to the respective terminal means. One or more fusible elements connected between the terminal means and disposed in a helical path on the core. The core comprises opening means that extend longitudinally thereof for a distance equal to at least one-half of a helical cycle of the fusible elements whereby leakage paths through the core are eliminated.

United States Patent Blewitt CORE CONSTRUCTION FOR CURRENT-LIMITING FUSE Donald'D. Blewitt, Pittsburgh, Pa.

Westinghouse Electric Corporation, Pittsburgh, Pa.

Filed: Oct. 17, 1973 Appl. N0.: 407,336

Inventor:

Assignee:

U.S. Cl 337/295, 337/159, 337/231 Int. Cl. l-lOlh 85/04 Field of Search 337/159, 231, 273, 276,

References Cited UNITED STATES PATENTS 12/1963 Jacobs 337/295 X 10/1971 Kozacka 337/159 l fi i l anwi [451 Feb. 25, 1975 Primary Examiner-J. D. Miller Assistant Examiner-Fred E. Bell Attorney, Agent, or Firm-L. P. Johns [57] ABSTRACT 7 Claims, 7 Drawing Figures hlgktk U ofii-tz LE PATENTEB FEBZ 5 I975 1 FlGS I1 FIG.7

Lil:

CORE CONSTRUCTION FOR CURRENT-LIMITING FUSE CROSS-REFERENCE TO RELATED APPLICATION This invention is related to that disclosed in the application of Donald D. Blewitt et al., Ser. No. 407,335, filed Oct. 17, 1973.

BACKGROUND OF THE INVENTION 1. Field Of The Invention This invention relates to current-limiting fuses and, more particularly, it pertains to a core construction therefor.

2. Description Of The Prior Art Current-limiting fuses, especially those for operation at high voltage, are provided with fusible elements that are wound spirally around a support or core which is disposed within a housing. of the. fuse. Examples of this type of construction are shown in U.S. Pat. Nos. 3,294,936 and 3,569,891. The fusible element ordinarily is composed of a wire or strap.

When a fuse interrupts a fault current, most of the energy is dissipated in melting the sand around the element. The higher the energy of interruption, the more sand that is melted. This process, brief as it is, generates temperatures of 2000C to 3000C. It is this band of intense heat surrounding the core which is responsible for core failures. At some level of input energy, depending on considerations of core construction and geometry, the intense heat degrades the insulating qualities of the core to the extent that a significant leakage current begins flowing through the fuse after an apparently successful interruption. The additional heating supplied by the leakage current further degrades the core, resulting in an increase in leakage current. The situation is unstable and ultimately results in total failure of the core and hence the fuse. The length of time to failure depends upon how hard the core is being worked. Samples have been observed in the laboratory to sustain voltage for eight minutes before exploding. Interestingly, present standards for this type of fuse require only that the fuse shall demonstrate voltage support for one minute.

The leakage'current path through the core may develop along the longitudinal edges of the core or along diagonal parallel lines created by core degradation during fusion of the spirally wound fusible elements. During fusion, temperatures of 2000C to 3000C are generated which causes the sand around the core to melt and form fulgurite and damage the core along paths nearest to the helical sections of the fusible element. As a result of the leakage paths, a supposedly open fuse may develop sufficient current leakage to cause the core to break down and the fuse to explode and thereby possibly damage equipment or cause injury to persons in the immediate area.

SUMMARY OF THE INVENTION It has been found in accordance with this invention that the foregoing problems may be overcome or reduced by providing a fuse structure comprising a generally tubular electrically insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, and axially-extending, electrically insulating core disposed in the casing with the ends disposed adjacent to and spaced from the respective terminal means, a fusible element disposed in a helical path on the core and connected between the terminal means, the core having longitudinally-extending slots and each slot having a length equal to at least one-half of a helical cycle of the fusible element. Another embodiment of the invention comprises slots having lateral portions extending from the longitudinal portion thereof, such as a T or L-shaped slot.

The advantages of the core construction of this invention is that the cut outs or openings in the core interrupt leakage paths which would otherwise develop, thus permitting higher ratings and improved reliability in electric fuses.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical, sectional view of a currentlimiting fuse structure embodying the invention;

FIG. 2 is a vertical, sectional view of another embodiment thereof;

FIG. 3 is a horizontal, sectional view taken on line III- -III of FIG. 2;

FIG. 4 is an elevational view of an electrically insulating core which forms part of the fuse structure shown in FIG. 1 and on which the fusible elements of the overall fuse structure are disposed;

FIG. 5 is a plan view of the member shown in FIG.

FIG. 6 is an elevational view of another embodiment of the electrically insulating support member; and

FIG. 7 is a plan view of the support member shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIGS. 1 and 2 of the drawings, a currentlimiting fuse structure is generally indicated at 10 and is particularly adapted for high-voltage applications, such as 2.8 kv and above. Inasmuch as the embodiment of both FIGS. 1 and 2 are substantially alike, only those parts which differ have different reference numbers. The fuse structure 10 includes a generally tubular casing or housing 12 which is formed from a suitable electrically insulating material that has sufficient structural strength to withstand the thermal conditions and internal pressures that may result during the operation of the fuse structure, such as glass-reinforced epoxy or melamine resin. Opposite ends of the casing 12 are closed off by similar terminal end caps or electrically conducting ferrules 14 and 16 which are secured in place by suitable means, such as the magnetic forming method which is described in detail in US. Pat. No. 3,333,336. Where desired, axially-extending, electrically conducting studs 18 and 20 may be mounted on or integrally formed with the ferrules l4 and 16, respectively, to permit the mounting of the fuse structure in particular types of supporting structures. The fuse structure 10 also comprises annular terminal members 22 and 24 that are disposed between the opposite ends of the casing 12 and the respective ferrules l4 and 16. Each of the terminal members 22 and 24 is formed from a suitable electrically conducting material, such as copper or a copper alloy, and includes a central opening with a tab portion or terminal 26 and 28 formed integrally at one side of the central opening which projects axially inwardly at one end of the associated casing.

In addition to the casing 12, the fuse structure comprises a fusible element 30 (FIG. 1) and 32 (FIG. 2) as well as an electrically insulating support member or core 34 (FIG. 1) and 36 (FIG. 2). The fusible elements 30 and 32 are separately mounted on the corresponding cores 34 and 36 and are disposed radially of the casings 12.

The fuse elements 30 and 32 are each electrically connected to and extend axially between the terminals 26 and 28. Each fuse element 30 and 32 comprises first and second end portions 30a and 30b, 32a and 32b, as well as intermediate portions 30c and 32c. As shown in FIG. 1, the fusible element 30 may be formed from a predetermined length of electrically conducting, fusible material, such as silver, of a flat, ribbon-type and including a plurality of axially spaced points 38 of reduced cross-sectional area which may be formed by V- notching the ribbon material from which the fusible element is formed on both sides. The series of restricted areas fuse initially during an interrupting operation of the fuse structure 10 to provide a series of space arcs. The sum of the voltages across the arcs result in a relatively high total are voltage during the operation of the fuse structure to limit the overload current which flows to a value less than that which would otherwise result. The fusible element 32 is provided with reduced areas and comprise a series of longitudinally spaced holes 40.

The intermediate portion 30c, 320 of the fusible elements 30, 32 may be formed by winding the intermediate portion of the fusible material from which the fusible element is formed on the corresponding cores 34, 36 so that the intermediate portions have a helical configuration as shown. The end portions 30a, 30b, 32a, 32b are in electrical contact with their corresponding terminals 26 and 28 and extend between the corresponding terminal members 22, 24 and ferrules 14 and 16 in an electrically conductive manner.

In accordance with this invention as shown in FIG. 4, the core 34 includes a plurality of longitudinally spaced opening means such as slots or cut outs 42. Each slot 42 extends longitudinally of the core 34 and preferably has a length equal to at least one-half a cycle of the helically wound fusible element 30; that is, the longitudinal distance on the core between adjacent front and back turns of the element. However, each slot 42 may have a greater length in order to provide a longer break or gap for the possible cuurrent leakage paths that would otherwise occur where the fusible element melts sand 44 which fills the casing 12. Thus, as indicated above, during fusion of the fusible element 30, the high temperatures incurred, upwards of 3000C, cause the sand adjacent the fusible element 30 to melt and degrade the insulating characteristics of the core, causing the core material to carbonize in diagonal sections corresponding to adjacent diagonal portions of the helical fusible element. The damaged core thereafter constitutes possible current leakage paths which are broken up by longitudinally-extending slots 42. In addition, the core 34 includes a plurality of longitudinally spaced edge notches 46 in which the fusible element 30 is wound and retained in place:

In the embodiment of the invention shown in FIGS. 6 and 7, opening means such as slots or cut cuts 48 having an L-shape are disposed at longitudinally spaced locations. The L-shaped slots 48 may be disposed along opposite edges of the core 36 for breaking up both edge and helical paths of current leakage. Thus, the longitudinally-extending portions of each slot 48 interrupt the possible diagonally-extending current leakage paths and the longitudinal portions of each slot provide the electrical barriers to the edge leakage paths or longitudinally-extending paths. In a manner similar to the core 34, the L-shaped slots 48 provide an electrical gap and help cool the core during fuse interruptions, thereby further reducing the thermal damage to the core. In addition, although the slots 48 have an L- shape, they may be of another configuration such as a T-shape.

Although the cores 34 and 36 are of a flat construction, they may be of a cruciform or star-shaped configuration. However, the planar form of core enables molding of the slots at the time of construction. The number of slots requiredand their dimensions is dependent uponthe design parameters of the fuse.

What is claimed is:

l. A fuse structure comprising a generally tubular, electrically insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, an axially-extending, electrically insulating support member disposed in said casing with the ends disposed adjacent to the respective terminal means, the support member comprising longitudinal portions and transverse intersupporting portions therefor, a fusible element disposed in a helical path on and around said longitudinal portions and connected between said terminal means, and the insulating support member comprising opening means at least a portion of which extends longitudinally of the member for a distance equal to at least one-half of a helical cycle of the fusible element.

2. The fuse structure of claim 1 in which the opening means comprises at least one elongated slot substantially parallel to the axis of said support member.

3. The fuse structure of claim 1 in which there are a plurality of elongated slots substantially coextensive with the support member.

4. The fuse structure of claim 3 in which the slots are axially disposed.

5. The fuse structure of claim 1 in which the opening means comprises at least one row of L-shaped slots, and each slot extending from an edge of said support member.

6. The fuse structure of claim 5 in which there are two rows of slots adjacent each edge of the said support member.

7. The fuse structure of claim 5 in which the portion of the L-shaped slots extend from one edge at locations between the cycles of the helically mounted fusible element. 

1. A fuse structure comprising a generally tubular, electrically insulating casing, terminal meaNs disposed adjacent to each of the opposite ends of said casing, an axially-extending, electrically insulating support member disposed in said casing with the ends disposed adjacent to the respective terminal means, the support member comprising longitudinal portions and transverse intersupporting portions therefor, a fusible element disposed in a helical path on and around said longitudinal portions and connected between said terminal means, and the insulating support member comprising opening means at least a portion of which extends longitudinally of the member for a distance equal to at least one-half of a helical cycle of the fusible element.
 2. The fuse structure of claim 1 in which the opening means comprises at least one elongated slot substantially parallel to the axis of said support member.
 3. The fuse structure of claim 1 in which there are a plurality of elongated slots substantially coextensive with the support member.
 4. The fuse structure of claim 3 in which the slots are axially disposed.
 5. The fuse structure of claim 1 in which the opening means comprises at least one row of L-shaped slots, and each slot extending from an edge of said support member.
 6. The fuse structure of claim 5 in which there are two rows of slots adjacent each edge of the said support member.
 7. The fuse structure of claim 5 in which the portion of the L-shaped slots extend from one edge at locations between the cycles of the helically mounted fusible element. 